Gold nanoclusters such as nanosphere and nanorod are of great potential in biological applications due to their small sizes, excellent biocompatibility and unique optical properties, including the well-characeterized surface plasmon resonance (SPR) phenomenon [1-6]. Gold nanoclusters have size-dependent SPR absorption in visible wavelength range; decreasing their sizes into less than 3 nm will induce the disappearance of the SPR and the emergence of photoluminescence [7-10]. The photoluminescence quantum yields of gold nanoclusters can be enhanced several orders of magnitude if their sizes are further decreased into the sub-nm scale [11-13]. This type of novel nanoclusters, termed as gold quantum dots (GQDs), have remarkable small size and excellent photoluminescent efficiency, which may be valuable properties toward their applicability in chemistry and biology [14,15].
To prepare GQDs with high quantum yield, it is essential to manipulate the nucleation of gold within a well-defined molecular scaffold. For example, thiolate-protected GQDs can be prepared by chemical reduction of the complex formed, from gold ions with alkanthiols, but the quantum yield is reduced because of their size-polydispersion since the gold core significant growth is influenced by the number of thiols [16-18]. Recently, polyamidoamine (PAMAM) dendrimer-encapsulated gold ions have been used to prepare GQDs with mono-dispersed size and excellent quantum efficiency [11-13]. Although the dendrimers are excellent molecular templates for the GQD formation, their safety still remain to be clarified prior to biological applications since there has been reported that they may have cytotoxcity such as inducing haemolysis of human red blood cells [19,20].
A heretofore unaddressed need exists in the art to address the deficiencies and inadequacies, especially in connection with dendrimer-encapsulated quantum dots.